Method of manufacturing a stainless high strength self-drilling screw

Abstract

A method for producing a high-strength self-drilling screw (100) made entirely of stainless steel is described, wherein the self-drilling screw (100) does not need to undergo a heat treatment process downstream of the manufacturing process to improve the material hardness. A shaft-shaped blank (110) is provided as a wire section (105) made of stainless steel. A screw head (140) is formed by cold forming at a first longitudinal end (120) of the blank (110). The diameter of an end portion of the blank is subsequently reduced by cold forming. A drill bit (150) is then produced at the second longitudinal end (130) of the blank (110) by a pinching movement transverse to the longitudinal axis. In a subsequent process step, the drill bit (160) is finished. Any projecting material lugs (170) remaining on the cutting edges (180) of the drill bit (160) are sheared off during the subsequent thread rolling process.During preforming, so-called contour edges (190) are formed in the area of ​​the future cutting edges (180), where the remaining material thickness is between 0.3 mm and 1.0 mm. The final forming of the drill bit is achieved by reshaping the contour edges (190) into cutting edges (180), with the remaining material thickness between the cutting edge (180) and the material flange (170) being between 0.05 mm and 0.2 mm. Local perforations are permitted.

Description

[0001] The present invention relates to a method for producing a high-strength fastener made of stainless steel, in particular a self-drilling screw, and to a self-drilling screw produced in this way. TECHNICAL BACKGROUND

[0002] Steel is typically defined as an iron-carbon alloy with a maximum carbon content of 2%. Carbon steel or unalloyed steel refers to variants that contain only minimal impurities or no deliberately added alloying elements such as chromium, nickel, copper, manganese, or silicon. Stainless steels, often referred to as special steels, are characterized by an alloy content of >10% chromium and less than 1.2% carbon.

[0003] Due to the incorporation of carbon into the matrix of the steel lattice, carbon steel is generally easier to harden than stainless steels. This hardening is typically achieved through hardening processes (heat treatment, case hardening such as carbonitriding) or cold forming. However, this – generally desirable – hardness of the steel comes at the fundamental disadvantage of susceptibility to corrosion. Therefore, certain end products require post-treatment (coating, passivation) to make them resistant to environmental influences within certain limits.

[0004] Among stainless steels, the most common are those with the alloying elements chromium and nickel, such as steel grades 1.4301 (V2A or A2) and 1.4401 (V4A or A4). Standards exist for these steels with specified compositions, so that these grades with comparable properties can be obtained from various sources. These steels are also referred to as austenitic because the alloying elements Ni, C, Mn, and N stabilize the austenite phase in the steel during production.

[0005] Duplex steel is a steel with a two-phase structure consisting of a ferrite matrix with islands of austenite. Compared to purely austenitic steels, duplex steel has a lower nickel content, which means that not the entire structure becomes austenitic at room temperature. Examples of this are the grades 1.4462 and 1.4362. Many everyday products as well as those used in the construction industry are made of steel; these include fasteners such as screws in all conceivable sizes for many applications. It is often desirable to combine the hardness of carbon steel with the corrosion resistance of stainless steel. This is usually achieved, particularly with self-drilling screws, by welding a section of carbon steel wire to a stainless steel wire of the same diameter and then processing this blank into a fastener in a known manner. The rolled or rolled steel can be used to weld a section of carbon steel wire to a section of stainless steel wire of the same diameter.The formed carbon steel tip can be hardened by heat treatment, while the stainless steel shaft can usually remain untreated, which also preserves the steel's corrosion-inhibiting properties.

[0006] In this document, a fastener is defined as a mechanical component with which two components can be permanently joined together (either detachably or permanently). A screw is specifically defined as a fastener that has an essentially longitudinally extended shaft with a cylindrical or cylinder-like cross-section. At one longitudinal end of the shaft there is a force application point, which can be designed as a head with force application surfaces. At the opposite end of the shaft is the tip of the screw. The shaft is provided with a thread in at least one section; it can be single-start or multi-start with a constant or variable pitch. The tip can be designed as a drill tip with cutting edges, as a blunt-conical, threadless displacement tip, or as a pointed-conical, self-hole- and thread-forming tip. Depending on the application, the thread can extend from the shaft to the cone or to the screw tip.

[0007] A self-drilling screw is a screw with a drill bit with cutting edges, in which the (shaft) thread is rolled and the drill bit is pinched. Pinching refers to a special type of cold forming in which the shank end of a blank is pressed into the desired shape of a drill bit by two tapered jaws. Other cold forming processes include rolling, upsetting, drawing, and extrusion. In general, experts understand cold forming to be the plastic deformation of metals below the recrystallization temperature, which is known to lead to work hardening of the formed material.

[0008] The production of fasteners and screws made entirely of stainless steel is well known; however, the penetration capacity of steel with corresponding (self-)drilling screws or hole- and thread-forming screws is limited. Bimetallic screws are complex and expensive to manufacture. Therefore, there is a need for fasteners, especially screws and self-drilling screws, that can be manufactured entirely from a corrosion-resistant steel grade and yet can penetrate steel sheets with thicknesses >1.5 mm without pre-drilling. STATE OF THE ART

[0009] German Patent Application DE 29 29 179 describes a corrosion-resistant, self-drilling, and thread-forming screw made of a stainless austenitic steel material (according to US Standard Series 300). The manufacturing steps include upsetting a head at the end of a wire section of the specified material and subsequently, at the opposite end, forming a drill point through a tightening process with a defined maximum closing speed of the tightening jaws. This transforms the austenitic structure of the drill point into a martensitic one. The document also recommends cooling the extruded blank to temperatures below 0°C, e.g., using dry ice.

[0010] Document EP 2 080 572 describes the production of a high-strength fastener made of austenitic steel of the 300 series (according to US standards). In a first step, a shaft blank is reduced in diameter by 15% through cold forming. The head and tip are then also produced by cold forming. The thread is created on the shaft by a rolling process. It is further proposed to improve the rust resistance of the cold-formed fastener through post-treatment or coating.

[0011] DE 2 103 053 and US 3,683,436 describe the production of a self-drilling screw with a pinched drill tip. The wire blank is reduced in diameter at one end by extrusion and then pinched into its final shape.

[0012] Document EP 2 617 500 A1 describes the production of a one-piece stainless steel drilling screw, in which the drill bit is manufactured in two forming steps. In a first step, an end section of a blank is flattened, and then the flattened end section is formed into the final drill bit.

[0013] It is an object of the present invention to improve the described methods, in particular to propose method steps for producing a corrosion-resistant, hole-forming and thread-forming screw with a drill tip, which largely makes the use of bimetallic screws superfluous.

[0014] This object is achieved by the features of the independent claim. Advantageous embodiments of the invention are specified in the dependent claims. DESCRIPTION OF THE INVENTION

[0015] The following describes the manufacturing process of a self-tapping screw according to the present invention as a sequence of process steps. These represent a sequential series of manufacturing steps that are typically carried out in close succession. Multi-stage cold forming machines are typically used for production, which use various tools to shape a workpiece into the desired shape in a timed manner and at defined forming rates. Intermediate steps in the process chain, such as quality control, transport, cleaning, sorting, and assembly, are not mentioned and have no influence on the feasibility of the invention.

[0016] The self-drilling screw according to the present invention is made entirely of stainless steel and achieves its high strength even without a heat treatment process following the manufacturing steps AF to specifically improve the material hardness. Such a process is therefore expressly excluded. The advantage lies in time and energy savings. Step A:

[0017] This process step involves preparing a wire section, which, as is known in the art, can be produced as a piece from a wire coil or as a section from corresponding stainless steel bar stock. This section is also referred to below as a blank. The section can be obtained from the starting material in a known manner by shearing, sawing, or other means. The length and diameter of the blank depend on the planned dimensions of the self-drilling screw to be produced; the design follows the known rules of the state of the art. Step B:

[0018] Upsetting a screw head by cold forming at a first longitudinal end of the blank. Upsetting the screw head can also be performed in one or more intermediate steps to control the degree of deformation per upsetting process. The creation of a force application point (external hexagon, hexalobular socket, Allen key, etc.) on or in the screw head is included in step B. Step C:

[0019] Reducing the diameter of a (longitudinal) section or shank section at the second longitudinal end of the blank by cold forming. A diameter reduction is performed at the second longitudinal end, opposite the screw head. This increases the surface hardness and lengthens the section in question. This diameter reduction can also be performed in several sub-steps; this variant is included equivalently in step C.

[0020] The length of the shank section will include at least the (longitudinal) section intended for manufacturing the drill bit in the subsequent steps D and E. However, if necessary, it may also include the shank section intended for the thread on the shank. The diameter reduction may also encompass the entire shank length up to the head, if this is technically feasible or useful. Step D:

[0021] Preforming a drill bit at the second longitudinal end of the blank by means of a pinching movement transverse to the longitudinal axis between two opposing tool jaws. The second longitudinal end of the blank, whose diameter was reduced in step C, is preformed. Preforming in this context means that two so-called pinching jaws, i.e., tool halves of a pinching device, preform the longitudinal end of the blank into a preform that is recognizable as a drill bit but does not correspond to the final dimensions.

[0022] The resulting preform encloses material flags created during this pressing process, which are formed from excess material displaced outward. The preformed drill bit is thus surrounded by a flat, thin, irregularly shaped material disc. The plane of the material disc corresponds to the closing plane or closing surface defined by the two tapered tool jaws.

[0023] Pre-forming the drill bit also means that the main cutting edges, cross cutting edges, chip flutes, and flanks of the future drill bit, depending on the selected drill bit layout, are identifiable in their shape and position, but do not yet correspond to the final dimensions. Typically, the tool jaws are designed so that the future cutting edges are at least partially embedded in the material disc and are already pre-defined by embossed contour edges during pre-forming. Step E:

[0024] Final forming of the drill bit. Through a further cold forming step, the main cutting edges, cross cutting edges, chip flutes, and flanks of the drill bit, as defined by the specific design, are finally formed in terms of shape and position, thus achieving the specified final dimensions. The contour edges are further formed and—depending on requirements—turned into cutting edges. During this step, the aforementioned material disc is further thinned and can be partially perforated or pre-perforated along the contour edges or the final-formed cutting edges. Step F:

[0025] The remaining protruding material tails on the cutting edges or contour edges of the drill bits are sheared off during a subsequent thread rolling process. The thread on the shaft can be applied along the entire length between the screw head and the drill bit or in partial areas, depending on the design.

[0026] The invention is characterized in steps D and E in that During pre-forming in step D, contour edges are formed in the area of ​​the future cutting edges, with the remaining material thickness between the contour edge and the material flag being between 0.3 mm (incl.) and 1.0 mm (excl.). These contour edges are recognizable as notches in the blank between the intended drill bit and the material flags. After finish-forming in step E, the contour edges are formed into cutting edges and the remaining material thickness between the cutting edge and the material flag is between 0.05 mm (incl.) and 0.3 mm (incl.), with local perforations also possible. This means that in certain areas the cutting edges are already separated from the material flags.

[0027] Preferably, the final forming of the drill bit in step E is carried out analogously to step D by a pinching movement of two opposing tool jaws transverse to the longitudinal axis of the blank. In order to achieve the described effect according to steps D and E and to simplify tool production, the tool jaws used in steps D and E are similar in shape, but not identical in design. Similar in shape is therefore to be seen as contrasting with the preforms known in the prior art, which merely provide for flattening, ablating, or a purely cylindrical raw shape. The tools used in step D, on the other hand, achieve a result that already resembles a drill bit, but would not yet be usable as such.

[0028] This design ensures that the final forming step, which gives the drill bit its final shape, remains a forming step with a lower degree of deformation, thus generating less forming heat. The achieved work hardening is retained in the cutting edges.

[0029] The forming heat generated during preforming can flow more easily into the surrounding material disc or material flags due to the only pre-stamped contour edges and reduces the tendency for recrystallization of the steel structure.

[0030] As mentioned at the beginning, the presented process allows for advantageous application with many stainless steel materials. These include stainless steels of the standards 1.4301, 1.4551, or 1.4307 (V2A), or 1.4401, 1.4571, or 1.4404 (V4A), or 1.4462, 1.4410, or 1.4501 (duplex). Similar steel grades from other standards are also included.

[0031] It is particularly advantageous to use stainless steel material as the starting material, which already has a surface strength of between 200 and 350 Hv before processing according to step A.

[0032] The diameter reduction explained in step C is advantageously at least 20% and at most 40%. The invention also encompasses forming in several sub-steps.

[0033] After the cold forming steps A to F described above, a coating advantageously follows to further improve the usability of the screw. A galvanic Zn-Ni coating has proven particularly effective. This process step is summarized under step G. The Zn-Ni coating from step G preferably contains 12-15% nickel.

[0034] In a further process step H, a single- or multi-layer anti-friction coating made of wax, plastics, or mixtures thereof can be applied to the Zn-Ni coating according to step G. Such coatings have a friction-reducing effect, particularly on the drill tip and its cutting edges.

[0035] A particularly effective additional step to ensure the high strength of the self-drilling screw described here is to cool the blank between pre-forming (step D) and final forming (step E) of the drill bit. It has been shown that both active cooling of the blank using a cooling fluid and passive cooling of the blank in a temperature-controlled environment are effective. The cooling fluid can be a liquid medium that is sprayed, squirted, or pumped onto the blank. It can also be an immersion process, a bath, or an equivalent cooling method. Gaseous media, cooling air, and ambient air cooled by dry ice are also effective.

[0036] Passive cooling is defined as the release of heat from the blank to the environment without any targeted cooling measures until the desired final temperature is reached. Cooling to normal ambient temperatures has been shown to be the least costly. In any case, cooling should be carried out to temperatures below 100°C, preferably below 50°C.

[0037] It has been shown that, thanks to the inventive method, the surface hardness of the blank after step C in the region formed thereby is essentially 300-380 Hv. If a cooling process as described above is carried out between steps D and E, the surface hardness of the blank after the cooling process can be increased (measured after step E in the region formed thereby essentially to 450-550 Hv).

[0038] A high-strength self-drilling screw can thus be manufactured in one piece from stainless steel using a process as described above. SHORT NAME OF THE FIGURE

[0039] Figure 1 shows a sequence of production stages A to F analogous to the described process steps. DESCRIPTION OF THE FIGURE

[0040] Figure 1 shows the sequence of the core process steps AF described in the invention.

[0041] A shows a wire section 105 with a first longitudinal end 120 and a second longitudinal end 130. This blank 110 receives a screw head 140 by cold forming in step B. The type of screw head 140 shown is representative of all types of technically feasible screw heads. C shows an (end) section 135 of blank 110 tapered by cold forming.

[0042] D shows the state of the blank 110 after preforming the drill bit 150. The contour edges 190 are shown as thick lines. These contour edges shown correspond to one possible way of forming a drill bit; a variety of these are known in the prior art. Where the contour edge 190 and the material flag 170 meet, the claim criterion "remaining material thickness" applies. Furthermore, any edge of the drill bit design that is formed is also considered a contour edge, even if it does not lie in the material disk or plane of the material flag. Logically, the claim criterion "remaining material thickness" is not applicable to these contour edges.

[0043] In area E of the Figure 1The state after final forming is shown with thin lines for the cutting edges 180, which emerged from the contour edges 190. The aforementioned claim criterion "remaining material thickness" applies to the cutting edges 180 that border the material flag 170. As can be seen, the material flag 170 has been enlarged by the second forming step. The permissible perforation is not shown here.

[0044] Area F of Figure 1 shows the finished cold-formed self-drilling screw 100 with thread 200 and drill bit 160 freed from the material flag 170.

[0045] In this section, the term blank is used as a collective term for all appearances of the self-drilling screw in steps A to (and including) E, even if the external appearance of the screw changes from step to step.

Claims

1. A method for producing a high-strength self-drilling screw (100) made entirely of a stainless steel material, wherein the self-drilling screw (100) does not undergo a heat treatment process downstream of the manufacturing process to improve the material hardness, with the following stepsA. Providing a shaft-shaped blank (110) as a wire section (105) made of a stainless steel material; B. Upsetting a screw head (140) by means of cold forming at a first longitudinal end (120) of the blank (110); C. Reducing the diameter of a section (135) at the second longitudinal end (130) of the blank (110) by cold forming; D. Preforming a drill bit (150) at the second longitudinal end (130) of the blank (110) by means of a pinching movement transverse to the longitudinal axis between two opposing tool jaws; E. Finish forming the drill bit (160); F. Shearing off any protruding material lugs (170) remaining on the cutting edges (180) of the drill bit (160) during a subsequent thread rolling process characterized in that- during preforming in step D, the area of ​​the later cutting edges (180) is formed into contour edges (190), wherein the remaining material thickness between the contour edge (190) and the material flag (170) is between 0.3 mm and 1.0 mm; and - after the final forming in step E, the contour edges (190) are formed into cutting edges (180) and the remaining material thickness between the cutting edge (180) and the material flag (170) is between 0.05 mm and 0.3 mm, wherein local breakthroughs are also possible.

2. Method according to claim 1, characterized in that the stainless steel material is a stainless steel selected from the standards 1.4301, 1.4551 or 1.4307 (V2A), or 1.4401, 1.4571 or 1.4404 (V4A) or 1.4462, 1.4362, 1.4410 or 1.4501 (Duplex).

3. Method according to claim 1-2, characterized in that the starting stainless steel material has a surface strength between 200 and 350 Hv before processing according to step A.

4. Method according to claim 1-3, characterized in thatthe diameter reduction in step C is at least 20% and at most 40%.

5. Method according to claim 1-4, characterized in that the final forming of the drill bit in step E is effected by a pinching movement of two opposing tool jaws transverse to the longitudinal axis of the blank.

6. Method according to claims 1-5, characterized in that the tool jaws used in steps D and E are similar in shape but not the same.

7. Method according to claims 1-6, characterized in that after the cold forming steps A to F, a coating step G takes place, whereby the coating is a galvanic Zn-Ni coating.

8. Method according to claim 7, characterized in that which the Zn-Ni coating from step G has 12-15% nickel.

9. Method according to claim 7-8, characterized in thatin a step H, a single- or multi-layer sliding coating made of wax, plastics or mixtures thereof is applied to the Zn-Ni coating according to step G.

10. Method according to claims 1-9, characterized in that A cooling process of the blank is carried out between pre-forming (step D) and final forming (step E) of the drill bit.

11. Method according to claim 10, characterized in that the cooling process is effected by actively cooling the blank using a cooling fluid.

12. Method according to claim 10-11, characterized in that the cooling process is carried out by passively cooling the blank in a temperature-controlled environment.

13. Method according to claims 1-12, characterized in that the surface hardness of the blank after step C in the area formed thereby is essentially 300-380 Hv.

14. Method according to claim 10-13, characterized in thatthe surface hardness of the blank after cooling and after step E in the area formed thereby is essentially 450-500 Hv.

15. High-strength self-drilling screw, made in one piece from stainless steel, by a process according to claims 1 to 14.

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